Bipolar electrode constructions are used to an ever increasing extent in order to obtain compact, efficient and economical electrolysis units and by bipolar electrode constructions are understood such electrodes which have one face functioning as an anode in one cell unit and the other face functioning as a cathode in an adjacent cell unit. By positioning a number of such units in a row, a battery of cells connected in series is obtained, which only requires current supply connections at the end electrodes but no special electrical connections between each cell unit. An especially advantageous electrode construction for these purposes, when a large electrode surface is desired, consists of a base plate provided with electrode plates fixed essentially at right angles to the base plate. This type of construction gives, besides a large electrode surface, a low voltage drop per running meter of the cell box which reduces risks of current leakage and short-circuit, both in the cell and outside it. The base plate can be provided with electrodes on one side only (unipolar embodiment) while means for current supply are provided on the other side. This arrangement does not only give the advantage that the base plates can form one wall of the cell, but also means that the base plates are available for a very simple connection to the current supply connections. The base plates can also be provided with electrode plates on both sides (bipolar embodiment) and the electrode plates on one side will then form cathodes while those on the other side will form anodes. By positioning a number of such electrode units in a row, with the electrode plates of adjacent electrodes between each other, a battery of cells connected in series will be obtained, after adding side walls, bottom and cover, and in this only the terminal electrode units have to be of the unipolar type and be provided with means for current supply on one side of the base plates instead of with electrode plates. This arrangement gives a very simple design of the cell row and also eliminates requirements on special current connections between the cells.
When employing the principles for bipolar electrodes at electrolytic production of chlorate, certain problems specific to this process will, however, arise. The chlorate production comprises a number of sub-steps and the sequence of reactions is probably a first formation of hydroxyl ions at the cathode during hydrogen generation and of elementary chlorine at the cathode, whereafter the hydroxyl ions and chlorine react to hypochlorite ions, which finally are disproportionated to chlorate and chloride. A number of requisites have to be fulfilled if this course of reaction is to take place at optimum conditions. The electrolyte flow past the electrode surfaces must be rather high and the production of hypochlorite at each circulation of the electrolyte past the electrodes must be moderate in order to avoid side reactions and other negative effects. The electrolyte circulation must further permit an efficient removal of formed hydrogen gas, and formed chlorine gas must be efficiently absorbed and retained in the electrolyte during the entire course of reaction. The electrolyte must be given a sufficient residence time after the electrolysis to give a complete reaction, particularly for the disproportionation of hypochlorite to chlorate. These conditions are seldom fulfilled in known bipolar electrode constructions of the above discussed type, as these generally are designed to be as compact as possible and to give the highest possible current density which often has resulted in an optimum relation between the volume and the residence time, but not in a carefully considered circulation flow of the electrolyte.
Other problems are caused by the corrosive environment in the cell. This environment is dependent partly on the composition of the electrolyte and partly on the high temperatures which are normally kept in order to achieve a high reaction rate. Corrosion problems lead to a limited service life for essentially all parts of the cell and necessitates a regular servicing of the cell. The cell should consequently be easy to dismount and it should be easy to replace and to clean its parts. As the electrodes themselves in cells with bipolar electrodes form partition walls problems of sealing these against the walls of the cell box will also arise, both with respect to electrolyte leakage and current leakage between the cells, and the sealings must be made with arrangements which are safe from the corrosion aspect and which do not make it much more difficult to disassemble the cell.
Known constructions do only to a limited extent possess the mentioned properties. Bipolar systems have thus been designed either with external reaction vessels and forced circulation, or they have had a complicated internal construction which has not been very suitable with respect to corrosion and maintenance.